1. Field of the Invention
This invention relates to a piezoelectric thin film element that uses a piezoelectric thin film having an alkali niobium oxide based perovskite structure.
2. Description of the Related Art
A piezoelectric substance is fabricated to various piezoelectric thin film elements according to various purposes, and particularly, is widely used as functional electronic parts such as an actuator to be deformed by application of voltage and a sensor to generate voltage due to the deformation of the piezoelectric thin film element adversely.
As the piezoelectric substance used for the actuator and the sensor, a dielectric substance including a lead based material having an excellent piezoelectric property, especially, a Pb(Zr1-xTix)O3 based ferroelectric substance of perovskite type called “PZT” has been widely used, and generally, it is manufactured by that an oxide including the respective constituent elements is sintered.
Currently, as various electronic parts become more downsized and upgraded, piezoelectric elements have been also strongly requested for the downsizing and upgrading. Piezoelectric materials fabricated by mainly using a sintering method which is a conventional method have a problem that as the thicknesses of the materials become thinner, especially, they becomes close to almost 10 μm, they become close to the crystal grain size constituting the materials, so that the influence thereof can not been ignored.
Therefore, a problem occurs that variation and deterioration of property become remarked, and recently a method of forming the piezoelectric substance to which a thin film technology and the like to take place of the sintering method is applied has been investigated in order to eliminate the problem. Nowadays, the PZT thin film formed by a sputtering method has been put into practice use as an actuator for a head of high-definition and high-speed ink-jet printer (Patent Literature 1).
On the other hand, a piezoelectric sintered body and a piezoelectric thin film made of the above-mentioned PZT includes approximately 60 to 70 weight % of lead oxide (PbO), so that it is not preferable in terms of ecological viewpoint and pollution control. Therefore, it is needed to develop a piezoelectric substance not containing lead in response to environment protection. Lately, various lead-free piezoelectric materials are investigated, and of the materials, potassium sodium niobate (the general formula: (K1-xNax)NbO3 (0<x<1)) is known (Patent Literature 2). This material of potassium sodium niobate is a material that has a perovskite structure and shows a relatively good piezoelectric property of the lead-free piezoelectric materials, so that it is expected as the best hope of the lead-free piezoelectric materials.    Patent Literature 1: JP-A-H09 (1997)-94955    Patent Literature 2: JP-A-2006-151796
However, although at present the material of potassium sodium niobate of the sintered body shows high piezoelectric constant (piezoelectric constant d31 is not less than −100 pm/V), the material of potassium sodium niobate of thin film realizes only less than half that of the sintered body.